1. Field of the Invention
The present invention relates to a wireless communication system with which communications are mutually made among a plurality of wireless stations as in a wireless local area network (LAN), and an apparatus and a method for a wireless communication. More particularly, the invention relates to a wireless communication system with which high speed data transmission is realized through multiplexing for a transmission line, and an apparatus and a method for wireless communication.
More specifically, the present invention relates to a wireless communication system with which high speed transmission is realized through a multiple input multiple output (MIMO) communication utilizing spatial multiplexing between a transmitter having a plurality of antennas and a receiver having a plurality of antennas, and an apparatus and a method for a wireless communication. More specifically, the invention relates to a wireless communication system with which signals are multiplexed and the multiplexed signals are transmitted without being influenced by crosstalk at all by using a singular value decomposition (SVD)-MIMO communication system utilizing the SVD of a channel information matrix H, and an apparatus and a method for a wireless communication.
2. Description of the Related Art
A wireless network attracts attention as a system for release from wirings in the traditional wired communication system. The Institute of Electrical and Electronics Engineers (IEEE) 802.11 and IEEE802.15 can be given as the standards for the wireless network.
A modulation system with which a communication speed of up to 54 Mbps is attained is supported based on the standard of IEEE802.11a/g. However, in recent years, the wireless LAN standards of the next generation with which the higher bit rate can be realized have been desired.
A multi-input multi-output (MIMO) communication attracts attention as one of the techniques for realizing the speeding up of the wireless communication. The MIMO communication is a communication system which includes a plurality of antenna elements on each of a sender side and a receiver side, and which realizes a stream obtained through spatial multiplexing. According to the MIMO communication system, a transmission capacity can be increased in correspondence to the number of antennas without increasing a frequency band, thereby attaining the increase in communication speed. In addition, utilization of the spatial multiplexing results in the excellent frequency use efficiency being obtained. The MIMO communication system is a communication system utilizing the channel characteristics, and thus is different from a simple transmitting and receiving adaptive array.
For example, IEEE802.11n as an extended standard of IEEE802.11a/g adopts an OFDM_MIMO system using an OFDM in primary modulation. Thus, the communication becomes possible at the transmission speed of 100 to 600 Mbps. An industry organization called Enhanced Wireless Consortium (EWC) which was organized on October, 2005 currently performs the development and promotion conforming to the IEEE802.11n specification with the MIMO as base.
The MIMO communication system is constructed as follows. That is to say, a channel information matrix H between a sender side and a receiver side is acquired by utilizing some kind of method. Moreover, the transmission signals spatially multiplexed in a phase of the transmission by using the channel information matrix H are spatially separated into a plurality of original streams in accordance with a predetermined algorithm.
The channel information matrix H is obtained as follows. That is to say, normally, the known training sequence is transmitted between a sender side and a receiver side through pairs of antennas. A channel transmission functions is estimated from a difference between the actually received signal and the known sequence. Also, the transmission functions for a combination of the sender side antennas and the receiver side antennas are arranged in the form of a matrix. When the number of sender side antennas is M, and the number of receiver side antennas is N, the channel information matrix becomes a matrix of N×M (row×column).
In addition, the method of spatially separating the received signals is roughly classified into two types. That is to say, one type is an open loop type in which the receiver carries out independently the spatial separation in accordance with the channel information matrix H. Also, the other type is a closed loop type in which the suitable beam formation is carried out for the receiver on the transmitter side as well by performing the sender side antenna weighting in accordance with the channel information matrix, thereby making an ideal spatial orthogonal channel. A singular value decomposition (SVD)-MIMO communication system utilizing the SVD of the channel information matrix H is known as one of the ideal forms of the closed loop type MIMO transmission.
FIG. 7 conceptually shows the SVD-MIMO communication system. In the SVD-MIMO communication system, the channel information matrix H having the channel information (transmission functions) corresponding to the pairs of antennas as elements is subjected to the singular value decomposition, thereby obtaining UDVH. Also, V is given as the sender side antenna weighting factor matrix, and UH is given as the receiver side antenna weighting factor matrix. Here, a superior H represents complex conjugate transpose.
Here, D represents a diagonal matrix having square roots of eigenvalues λi of a covariance matrix A of the channel information matrix H as diagonal elements (a suffix i means an i-th spatial stream). Also, the eigenvalues λi correspond to qualities of the corresponding spatial streams, respectively. The singular values λi are arranged in the order of decreasing the value of the diagonal element of the diagonal matrix D, and a power ratio distribution corresponding to the communication quality represented by the magnitude of the singular value, and an allocation of the modulation system are carried out for the streams. As a result, it is possible to realize a plurality of theoretically independent transmission lines for which the spatial orthogonal multiplexing is carried out. Thus, the receiver side can take out a plurality of original signal sequences without being influenced by the crosstalk at all, and the highest performance can be theoretically attained.
In the example shown in FIG. 7, the transmitter includes M transmission antennas. Thus, the transmitter distributes the transmission data to the K transmission streams, multiplexes the transmission data through the spatial/time encoding, and distributes the multiplexed transmission data to the transmission antennas, respectively, thereby transmitting the multiplexed transmission data through the respective channels. A transmission signal x′ at this time is expressed in the form of a vector of M×1. On the other hand, the receiver includes the N reception antennas. Thus, the receiver subjects a received signal y′ expressed in the form of a vector of N×1 to the spatial/time decoding, thereby obtaining the received data composed of the K reception streams without the crosstalk among the streams. The channel information matrix in this case is expressed in the form of a matrix H of N×M. Also, the spatial streams having only the number which is less one (MIN[M, N]) of the number of sender side antennas, and the number of receiver side antennas are ideally formed.
An element hij of the channel information matrix H is the transmission function from the j-th transmission antenna to the i-th reception antenna (where i is a positive integral number of 1 to N, and j is a positive integral number of 1 to M). Also, the received signal vector y′ is expressed by the following expression (1) in which a noise vector n is added to the product of the transmission signal vector and the channel information matrix.y′=Hx′+n  (1)
When being subjected to the singular value decomposition in the manner as described above, the channel information matrix H is expressed by the following expression (2):H=UDVH  (2)
Here, the sender side antenna weighting factor matrix V, and the receiver side antenna weighting factor matrix UH are unitary matrices which meet the following expressions (3) and (4), respectively:UHU=I  (3)VHV=I  (4)
Where I represents a unit matrix.
That is to say, a matrix in which the normalized eigenvectors of HHH are arranged is the receiver side antenna weighting factor matrix UH. Also, a matrix in which the normalized eigenvectors of HHH are arranged is the sender side antenna weighting factor matrix V. In addition, D is the diagonal matrix, and has square roots of the eigenvalues λ of HHH or HHH as the diagonal components. In other words, when smaller one of the number, M, of sender side antennas, and the number, N, of receiver side antennas is L(=min (M, N)), the diagonal matrix D becomes a square matrix of L×L as expressed by the following expression (5):
                    D        =                  [                                                                                          λ                    1                                                                              0                                            …                                            0                                                                    0                                                                                  λ                    2                                                                              …                                            0                                                                    ⋮                                            ⋮                                            ⋱                                            ⋮                                                                    0                                            0                                            …                                                                                  λ                    L                                                                                ]                                    (        5        )            
In the system shown in FIG. 7, the transmitter carries out the weighting transmission by using the antenna weighting factor matrix V in the phase of the transmission. On the other hand, the receiver carries out the weighting reception by using UH as the antenna weighting factor matrix in the phase of the reception. Therefore, since each of the matrices U and V is the unitary matrix (U is the matrix of N×L, and V is the matrix of M×L), the received signal y is expressed by the following expression (6):
                                                                           y                =                                ⁢                                                      U                    H                                    ⁢                                      y                    ′                                                                                                                          =                                ⁢                                                      U                    H                                    ⁡                                      (                                                                  Hx                        ′                                            +                      n                                        )                                                                                                                          =                                ⁢                                                      U                    H                                    ⁡                                      (                                          HVx                      +                      n                                        )                                                                                                                          =                                ⁢                                                                                                    U                        H                                            ⁡                                              (                                                  UDV                          H                                                )                                                              ⁢                    Vx                                    +                                                            U                      H                                        ⁢                    n                                                                                                                          =                                ⁢                                                                            (                                                                        U                          H                                                ⁢                        U                                            )                                        ⁢                                          D                      ⁡                                              (                                                                              V                            H                                                    ⁢                          V                                                )                                                              ⁢                    x                                    +                                                            U                      H                                        ⁢                    n                                                                                                                          =                                ⁢                                  IDIx                  +                                                            U                      H                                        ⁢                    n                                                                                                                          =                                ⁢                                  Dx                  +                                                            U                      H                                        ⁢                    n                                                                                                            (          6          )                    
Here, the received signal y and the transmission signal x are not expressed in the form of vectors depending on the number of sender side antennas, and the number of receiver side antennas, respectively, but are expressed in the form of (L×1) vectors, respectively. The transmission signal for each stream can be received without being influenced by the crosstalk at all because D is the diagonal matrix. Also, since the diagonal elements of the matrix D become the square roots of the eigenvalues λi, the electric power of each of the received signals is proportional to λi. In addition, for the noise component n, each of the columns of U has the eigenvectors each having a norm normalized to 1. Hence, UHn changes no noise electric power. With respect to the size, UHn becomes a (L×1) vector, and y and x have the same size.
As described above, in the SVD-MIMO transmission, a plurality of independent and logical paths each of which is free from the crosstalk can be obtained in spite of the same frequency and the same time. That is to say, a plurality of data can be transmitted through the wireless communication by using the same frequency at the same time. As a result, it is possible to realize the improvement in the transmission speed.
As have been described above, the antenna weighting method in the transmitter, especially, the weighting method for the SVD-MIMO transmission (eigenmode transmission) can be expressed in the form of a mathematical expression. It is necessary for the normal actual equipment to execute the processing in real time by using an arithmetically operating circuit which is structured with the realistic circuit scale. For this reason, the integer arithmetic operation is carried out for the calculation for obtaining the weighting factor matrix from the channel information matrix H. Unlike the real number arithmetic operation, the integer arithmetic operation causes a problem such as an increase in arithmetic operation error, an overflow, an underflow or the like due to an influence of a word length limitation. As a result, there is the high possibility that the row norm of the weighting factor matrix largely varies.
On the other hand, an upper limit is generally set in the transmission output of the wireless transmitter from the Radio Law control or the like. Here, normally, the transmission is carried out with the output as large as possible in the range of not exceeding the upper limit because the transmission output is connected directly with a communication distance. In the transmitter as well which carries out the weighting transmission described above, the matrix V having the row norm normalized to 1 is used as the weighting factor matrix, thereby preventing the transmission output from fluctuating for any weighting factor matrix (that is, thereby preventing the transmission output from exceeding the upper limit).
However, in the case where the arithmetic error, the overflow or the like accompanying the integer arithmetic operation or the like as described above occurs in the process for arithmetically operating the weighting factor matrix, it is possible that the weighting factor matrix becomes the unexpected value. Thus, it is the possibility that the transmitter multiples the transmission signal by the transmission weighting factor matrix having the unexpectedly large value, so that the output of the resulting signal exceeds the upper limit of the transmission output, thereby running foul of the Radio Law control.
The technique as described above, for example, is disclosed in Japanese Patent Laid-Open No. 2005-160030.